The present invention relates to a microprogram control system and, more particularly, to a control system for executing microinstructions.
A microprogram control system is a typical system for controlling the operation and status of various logic units such as central processing units and input/output devices. In such a microprogram control system, a microprogram routine corresponding to a user instruction (referred to as a macroinstruction hereinafter) is executed.
FIG. 1 is a schematic block diagram of a microprogram control system according to the prior art.
Referring to FIG. 1, microinstructions for designating the kind of control to be performed, the subject of the control, the control procedure and so on are stored according to a predetermined format in respective address memory locations of a control memory 1. The memory locations in the control memory 1 are designated by respective addresses in a memory address register 4 under the control of a memory address control circuit 3, and the microinstructions in locations designated by predetermined addresses are sequentially read out of the control memory to a microinstruction register 2. The microinstructions read out in this manner are decoded into a series of control signal sequences by a decoder 5 which controls the operation and status of the logic units. The part of a next address designating a memory location in the control memory is input to the memory address control circuit 3 which then specifies the address of the control memory 1 to be accessed next according to various conditions. In order, however, to effect a jump to routine in a program for executing other processes with certain data (e.g., macroinstructions), a step was required for reading out and decoding the data (the operation code (OP code) of the macroinstruction). This resulted in various disadvantages usually caused by the increase in the number of steps, which resulted in an increase in the required control memory capacity and a reduction in processing speed.